This invention relates to a method and apparatus for reducing feedback in acoustic systems, particularly hearing aids. More specifically, the invention relates to hearing aids that employ digital processing methods to implement hearing loss compensation and other forms of corrective processing, and is concerned with reduction of acoustic feedback in such hearing aids.
Acoustic feedback in hearing aids occurs because the gain and phase of the acoustic path from the receiver to the microphone are such that a feedback signal arrives at the microphone in phase with the input signal and with a magnitude that is greater than or equal to the input signal. This problem is especially prevalent in high-power hearing aids. A number of methods have been developed in the past for acoustic feedback reduction in digital hearing aids. Recently, techniques that use digital signal processing have been proposed.
Kates, J. (Feedback Cancellation in Hearing Aids: Results from a Computer Simulation, IEEE Trans. on Acoustics Speech and Signal Processing, 1991, 39:553-562) implemented a scheme where the open-loop transfer function of the hearing aid is estimated by opening the forward signal path of the hearing aid and injecting a short-duration (50 ms) noise probe signal. Because the probe signal is very short in duration, it is inaudible to the hearing aid user. (It may, however, reduce the intelligibility of the processed speech signal.) When acoustic feedback is detected, the forward path is opened, the noise signal is injected and an adaptive filter is adjusted to estimate the transfer function of the feedback path and eliminate the acoustic feedback. Computer simulations demonstrated that this scheme provides the potential for 17 dB of feedback cancellation. A more recent scheme proposed by Maxwell, J. and Zurek, P. (Reducing Acoustic Feedback in Hearing Aids, IEEE Trans. on Speech and Audio Processing, Vol. 3, No. 4, pp. 304-313, July 1995) is similar in operation except that it adapts during the xe2x80x9cquietxe2x80x9d intervals of the input speech signal, as well as adapting when feedback is detected.
Dyrlund, O. and Bisgaard, N. (Acoustic Feedback Part 2: A Digital Feedback System for Suppression of Feedback, Hearing Instruments, Vol. 42, No. 10, pp. 44-25, 1991); and Dyrlund and Bisgaard (Acoustic Feedback Margin Improvements in Hearing Instruments Using a Prototype DFS (digital feedback suppression) System, Scand Audiology, Vol. 20, No. 1, pp. 49-53, 1991) developed a scheme that was implemented in a commercial hearing aid, the Danavox DFS. This scheme continuously characterizes the acoustic feedback path with an injected noise signal. If feedback is detected, the DFS algorithm injects a cancellation signal into the hearing instrument signal path that is at the same frequency but has opposite phase to the feedback signal. This scheme can provide 8-15 dB higher gain than a hearing aid without feedback reduction. However, it has the disadvantages that the injected noise signal may be audible for some listeners and that the noise signal may mask some speech cues at higher frequencies.
The present invention provides a feedback scheme which uses a filtered noise source that is passed through a shaping filter whose frequency response is dependent on the spectrum of the input signal and a simplified model of the human auditory system. If the filter is adapted in a known manner [Jayant, N., Johnson J., and Safranek, R., Signal Compression Based on Models of Human Perception, Proc. of IEEE, Vol. 81, No. 10, pp. 1385-1422, October 1993] the shaped noise signal that is added to the hearing aid input signal (at a relatively low signal-to-noise ratio of 15 dB or greater) will be inaudible to the hearing aid wearer. This inaudibly shaped noise source is used continuously to characterize the acoustic feedback path. If feedback is detected, adjustments are made in the hearing aid frequency response to eliminate it.
In accordance with the present invention, there is provided a method of controlling feedback in an acoustic system having an input for an acoustic input signal and output signal that generates a potential feedback path between the output and the input, the method comprising the steps of:
(1) generating a first input signal from the acoustic input signal and making a spectral estimate of the first input signal;
(2) subjecting the spectral estimate to a psycho-acoustic model to generate a control signal;
(3) passing a noise signal through a shaping filter and controlling the shaping filter with the control signal, to generate frequency-shaped noise, which is inaudible to someone hearing the acoustic output signal;
(4) adding the frequency-shaped noise to the first input signal to form a combined signal;
(5) processing the combined signal in a forward signal path having a transfer function, to generate a first output signal;
(6) analyzing the first output signal and the frequency-shaped noise signal, to determine the presence of feedback at different frequencies;
(7) using the first output signal to generate the acoustic output signal; and
(8) modifying the transfer function of the forward signal path, to reduce the gain thereof at frequencies where feedback is detected.
Preferably, in step (2), the psycho-acoustic model selected from one of a normative psycho-acoustic model and a measured psycho-acoustic model representative of the hearing characteristics of an individual.
In a further embodiment of the present invention, step (6) comprises forming a cross-spectral estimate between the first output signal and the frequency-shaped noise and an auto-spectral estimate for the frequency-shaped noise, dividing the cross-spectral estimate by the auto-spectral estimate to obtain a spectral ratio, and determining when the frequency response of the spectral ratio varies from the frequency response of the forward path, indicative of feedback.
The method of the present invention can be applied to any suitable acoustic system, for example a digital hearing aid or a public address system.
In another embodiment of the present invention, steps (3) and (6) are based on maximum length sequence methods, such that step (3) comprises taking the fast Hadamard transform of the control signal to generate the frequency-shaped noise, and step (6) comprises taking the fast Hadamard transform of the first output signal from the forward path, generating the power spectrum of the fast Hadamard transform of the first output signal and the power spectrum of the fast Hadamard transform of the control signal, and dividing the two power spectrums to obtain a spectral ratio from which feedback can be detected.
The present invention also provides apparatus corresponding to the method aspects just defined. The apparatus is for processing an acoustic signal and generating an acoustic output, and the apparatus comprises:
an input means for receiving an acoustic input signal and for generating a first input signal;
an output transducer for generating an output acoustic signal;
a forward signal path within the apparatus connecting the input means to the receiver and having a main transfer function for generating a first output signal;
a feedback path between the receiver and the input means enabling at least a portion of the output acoustic signal to be received at the input means;
a spectral estimation means connected to the input means for receiving the first input signal and for generating a spectral estimate of the acoustic input signal;
a psycho-acoustic model means connected to the spectral estimation means for forming a control signal from the spectral estimate;
a noise generation means connected to the psycho-acoustic model means for generating a noise signal whose spectrum is dependent upon the control signal;
means for adding the noise signal to the first input electrical signal to form a combined signal, for processing in the forward signal path; and
means for analyzing the noise signal and the combined signal after processing in the forward signal path to determine the presence of feedback and for modifying the main transfer function of the forward path to eliminate any substantial acoustic feedback.